Now is the Time to Control and Manage Waterhemp in Corn and Soybean Fields

Source: University of Minnesota Extension | May 13, 2011

Waterhemp is an annual weed species in the pigweed family capable of producing greater than 1 million seeds/plant and due to a limited number of effective herbicides[3], especially in sugarbeet and soybean[4], is difficult to control compared to most weed species. In addition to the production of large quantities of seeds, continual germination throughout the growing season and an increased frequency of herbicide-resistant biotypes adds to the degree of difficulty in keeping this weed species under control. The good news is that the longevity of waterhemp seeds in the seedbank is relatively short compared to most species (1-12% survival after four years), meaning complete control (zero seed production) of all plants over a three to four year time period should significantly reduce the waterhemp seed bank densities, allowing the farmer to take control of this difficult weed problem.

Why is Proper Waterhemp Management Important?

Glyphosate-resistant waterhemp has recently been confirmed in a small area of Richland County, North Dakota. Glyphosate-resistant waterhemp was confirmed in southern Minnesota in 2007 and has continued to increase over time. In 2010, based on visual observations, a high percentage of fields in Renville and Swift counties in Minnesota had some frequency (> 1 plant/field) of glyphosate-resistant waterhemp present at harvest. The frequency of glyphosate-resistant waterhemp has continued to increase due to the continued planting of Roundup Ready[5] crops and the exclusive use of glyphosate.

Where glyphosate-resistant biotypes are known to be present in fields, waterhemp must be properly managed using reactive management strategies. Proactive management strategies will be necessary in other fields in order to attempt to prevent glyphosate-resistant biotypes from developing. Often the level of weed-control inputs will be lower in fields where proactive strategies are employed due to lower weed seed bank populations. Proper management requires managing waterhemp across the entire cropping system over time. Many growers have begun using pre-emergence residual herbicides. Increasing crop rotation diversity in the cropping system and focusing on the use of Roundup Ready crops in the rotation where the fewest alternative herbicides to glyphosate exist will also reduce the selection pressure for glyphosate-resistant waterhemp. To illustrate, wheat, corn[6] and LibertyLink corn and soybean provide more chemical weed control options and should be considered.

Follow label directions for herbicide rates according to soil type and weed size, use of products by geography and crop rotation. Consult the publication PRE and POST Herbicide Diversification Options[7] for information regarding crop rotation and herbicide effectiveness. Scout fields following the first postemergence application to determine need for a second application. If planting no-tillage[8] corn or soybean, apply the necessary preplant herbicides to control emerged waterhemp and other weeds prior to planting and use the appropriate adjuvant(s) and rate(s) for all products in the preplant treatment. In areas of low rainfall, incorporation of preemergence herbicides may improve waterhemp control but may increase crop-injury for some products.

Proper proactive and reactive management of waterhemp this growing season and into the future is necessary to reduce selection of herbicide resistant biotypes and maintain the effectiveness of Roundup Ready crops in the rotation. This is especially true for Roundup Ready sugarbeet in which few herbicides are available to effectively manage waterhemp.